The present invention relates to intracellular receptors, nucleic acids encoding same, and uses therefor. In a particular aspect, the present invention relates to methods for the modulation of physiological response to elevated levels of steroids and steroid-like compounds.
Nuclear receptors constitute a large superfamily of ligand-dependent and sequence-specific transcription factors. Members of this family influence transcription either directly, through specific binding to the promoters of target genes (see Evans, in Science 240:889-895 (1988)), or indirectly, via protein-protein interactions with other transcription factors (see, for example, Jonat et al., in Cell 62:1189-1204 (1990), Schuele et al., in Cell 62:1217-1226 (1990), and Yang-Yen et al., in Cell 62:1205-1215 (1990)). The nuclear receptor superfamily (also known in the art as the xe2x80x9csteroid/thyroid hormone receptor superfamilyxe2x80x9d) includes receptors for a variety of hydrophobic ligands, including cortisol, aldosterone, estrogen, progesterone, testosterone, vitamin D3, thyroid hormone and retinoic acid, as well as a number of receptor-like molecules, termed xe2x80x9corphan receptorsxe2x80x9d for which the ligands remain unknown (see Evans, 1988, supra). These receptors all share a common structure indicative of divergence from an ancestral archetype.
Lipophilic hormones such as steroids, retinoic acid, thyroid hormone, and vitamin D3 control broad aspects of animal growth, development and adult organ physiology. The effects of these hormones are mediated by members of the nuclear receptor superfamily. The nuclear receptors for such non-steroidal compounds as thyroid hormone (TR), vitamin D3 (VDR), all-trans retinoic acid (RAR), fatty acids and eicosanoids (PPAR) form heterodimers with the 9-cis retinoic acid receptor (RXR) that bind bipartite hormone-response elements (HREs) composed of directly repeated half sites related to the sequence AGGTCA (see, for example, Mangelsdorf and Evans in Cell 83:841 (1995)).
In contrast, nuclear receptors for steroidal compounds function as homodimers and bind to palindromic target sequences spaced by three nucleotides (see, for example, Beato et al. in Cell 83:851 (1995)). In addition to the known receptors, a large group of structurally-related xe2x80x9corphanxe2x80x9d nuclear receptors has been described which possess obvious DNA and ligand binding domains but lack identified ligands (see, for example, Mangelsdorf et al., in Cell 83:835 (1995); Enmark and Gustafsson in Mol. Endocrinol. 10:1293 (1996); and O""Malley and Conneely in Mol. Endocrinol. 6:1359 (1992)). Each has the potential to regulate a distinct endocrine signaling pathway.
It is widely viewed that the hormone response is a consequence of the release, from an endocrine gland, of a ligand that circulates through the blood, and coordinately regulates responses in target tissues by acting through specific nuclear receptors. Hormone responsiveness is dependent on the ability to rapidly clear ligand from the blood and the body so that, in the absence of a stimulus, target tissues return to a ground state. Hormonal homeostasis is thus achieved by the coordinated release and degradation of bioactive hormones.
Steroid hormones and their many metabolites are primarily inactivated by reduction and oxidation in the liver. Since literally hundreds of adrenal steroids have been identified (e.g., dozens of each of the sex steroids (androgens, estrogens and progestins), 25-35 vitamin D metabolites, and likely hundreds of fatty acids, eicosanoids, hydroxyfats and related bioactive lipids), the problem of efficient ligand elimination is critical to physiologic homeostasis. In addition to the existence of a myriad of endogenous hormones, a similar diversity of ingested plant and animal steroids and bioactive xenobiotic compounds must also be degraded.
Selye first introduced the concept that exogenous steroids and pharmacologic substances may function to modulate the expression of enzymes that would protect against subsequent exposure to toxic xenobiotic substances (see Selye in J. Pharm. Sci. 60:1 (1971)). These compounds, which Selye called xe2x80x9ccatatoxic steroidsxe2x80x9d are typified by the synthetic glucocorticoid antagonist, pregnenolone-16-carbonitrile (PCN). PCN, and a variety of xenobiotic steroids, induce the proliferation of hepatic endoplasmic reticulum and the expression of cytochrome P450 genes (see, for example, Burger et al., in Proc. Natl. Acad. Sci. (USA) 89:2145 (1992); Gonzalez et al., in Mol. Cell. Biol. 6:2969 (1986); and Schuetz and Guzelian in J. Biol. Chem. 259:2007 (1984)). One consequence of such regulation is the induction of nonspecific xe2x80x9cprotectionxe2x80x9d against such diverse xenobiotic compounds as digitoxin, indomethacin, barbiturates and steroids.
Insight into the mechanism by which PCN exerts its catatoxic effects is provided by the demonstration that PCN induces the expression of CYP3A1 and CYP3A2, two closely related members of the P450 family of monooxygenases (see, for example, Elshourbagy and Guzelian in J. Biol. Chem. 255:1279 (1980); Heuman et al., in Mol. Pharmacol. 21:753 (1982); Hardwick et al., in J. Biol. Chem. 258:10182 (1983); Scheutz and Guzelian in J. Biol. Chem. 259:2007 (1984); Scheutz et al., in J. Biol. Chem. 259:1999 (1984); and Gonzalez et al., in J. Biol. Chem. 260:7435 (1985)). The CYP3A hemoproteins display broad substrate specificity, hydroxylating a variety of xenobiotics (e.g., cyclosporin, warfarin and erythromycin), as well as endogenous steroids (e.g., cortisol, progesterone, testosterone and DHEA-sulfate. See, for example, Nebert and Gonzalez in Ann. Rev. Biochem. 56:945 (1987) and Juchau in Life Sci. 47:2385 (1990)). A PCN response element (which is highly conserved in the CYP3A2 gene promoter) has since been identified in subsequent studies with the cloned CYP3A1 gene promoter (see Miyata et al., in Archives Biochem. Biophysics 318:71 (1995) and Quattrochi et al., in J. Biol. Chem. 270:28917 (1995)). This response element comprises a direct repeat of two copies of the nuclear receptor half-site consensus sequence AGTTCA.
In addition to inducing CYP3A gene expression, PCN has also been shown to have marked effects on hepatic cholesterol homeostasis. These effects include significant decreases in the levels of HMG-CoA reductase and cholesterol 7xcex1-hydroxylase gene expression, with associated reductions in sterol biosynthesis and bile acid secretion. PCN has also been reported to enhance the formation of cholesterol esters and the hypersecretion of cholesterol into the bile. Thus, PCN affects key aspects of cholesterol metabolism, including its biosynthesis, storage and secretion.
While it appears that catatoxic steroids regulate the expression of cytochromes and other detoxifying enzymes, two lines of evidence argue that such regulation is independent of the classical steroid receptors. First, many of the most potent compounds (e.g., PCN, spironolactone, cyproterone acetate) are steroid receptor antagonists, whereas other potent compounds (e.g., dexamethasone) are receptor agonists (see, for example, Burger et al., supra). Second, the nonspecific protective response remains after bilateral adrenalectomy (and presumably in the absence of most adrenal steroids) but not after partial hepatectomy (see, for example, Selye, supra)
Activation of orphan nuclear receptor(s) by catatoxic steroids provides a possible mechanism for the induction of xenobiotic metabolizing enzymes by compounds that do not activate known steroid receptors. Because such enzymes are activated by high (pharmacological) doses of xenobiotic and natural steroids, such a xe2x80x9csensorxe2x80x9d would be expected to be a broad-specificity, low-affinity receptor. Such receptors could be activated not only by endogenous steroids and metabolites but also by exogenous compounds such as phytosteroids, xenobiotics and pharmacologic inducers. Indeed, it is known that a variety of such compounds can activate P450 genes responsible for their detoxification or degradation (see, for example, Fernandez-Salguero and Gonzalez in Pharmacogenetics 5:S123 (1995); Denison and Whitlock, Jr. in J. Biol. Chem. 270:18175 (1995); Hankinson in Ann. Rev. Pharmacol. Toxicol. 35:307 (1995); and Rendic and Di Carlo in Drug Metab. Rev. 29:413 (1997)).
Accordingly, there is still a need in the art for the identification and characterization of broad specificity, low affinity receptors which participate in the mediation of the physiological effect(s) of hormones.
In accordance with the present invention, we have isolated and characterized an example of a novel class of nuclear receptor(s), termed the steroid X receptor (SXR). SXR is expressed almost exclusively in the liver, the primary site of xenobiotic and steroid catabolism. Unlike classical steroid receptors, SXR heterodimerizes with RXR and binds to directly repeated sequences related to the half-site, AGTTCA. SXR can activate transcription through response elements found in some steroid inducible P450 genes in response to a wide variety of natural and synthetic steroid hormones, including antagonists such as PCNxe2x80x94ideal properties for a xe2x80x9csteroid sensing receptorxe2x80x9d which mediates the physiological effect(s) of hormones. SXR represents the first new class of steroid receptors described since the identification of the mineralocorticoid receptor ten years ago.
In accordance with a particular aspect of the present invention, there are also provided nucleic acid sequences encoding the above-identified receptors, as well as constructs and cells containing same, and probes derived therefrom. Furthermore, it has also been discovered that a wide variety of substrates modulate the transcription activating effects of invention receptors.